1. Field of the Invention
This invention relates to elastomeric copolymers of 1,3-butadiene and 1,3-pentadiene (piperylene) in which the monomers comprising the copolymer are randomly distributed in the polymer chain in a stereoregulated manner. More particularly, this invention relates to copolymers of said monomers prepared with the aid of an iodine-containing catalyst system comprised of a titanium halide, a metal iodide from Groups IIIa or IVa of the Periodic Table of the Elements an aluminum alkyl, elemental iodine, and a Lewis Base. The vulcanized polymers possess a high abrasion resistance and are particularly useful in automotive tire-tread compositions.
2. Prior Art
The preparation of crystalline copolymers of butadiene and piperylene was first reported by G. Natta et al in Makromol. Chem. 53, 52 (1962). Two types of copolymer were disclosed. In the one, prepared with a heterogeneous catalyst system consisting of Al(Et).sub.3 -VCl.sub.3, both monomers were exclusively enchained in a 1,4-trans-configuration. In the other, using a homogeneous catalyst system consisting of Al(Et).sub.2 Cl and vanadium tris-acetylacetonate, all of the butadiene was enchained as the 1,4-trans-stereoisomer while the piperylene monomer units were either enchained as the 1,2- or 1,4-trans-stereoisomers.
In a subsequent publication in Makromol. Chem. 61, 90 (1963), L. Porri et al, described the preparation of copolymers of butadiene and piperylene with homogeneous catalyst systems consisting of (1) Al(Et).sub.2 Cl-V(AcAc).sub.3 and (2) Al(Et).sub.2 Cl-VCl.sub.3.3THF in which (AcAc) is an acetylacetonyl ion and THF is tetrahydrofuran. The products from both catalyst systems were identical and were characterized by the absence of any pendant vinyl unsaturation; 100 percent enchainment of the butadiene as the 1,4-trans-isomer; 100 percent enchainment of the piperylene as either the 1,2 or 1,4-trans-isomer and the complete absence of any cis-isomers in the polymers.
A. Carbonaro et al in an article in Rubber Chem. & Tech. 46, 1274 (1973 ) have described the synthesis and vulcanizate properties of a series of butadiene-piperylene copolymers prepared with a catalyst system consisting of a mixture of Al(Et).sub.2 Cl with AlEtCl.sub.2 and V(AcAc).sub.3. The stereoisomeric configuration of the butadiene and piperylene in the copolymers was essentially the same as in the copolymers prepared by Porri et al with similar homogeneous catalyst systems having the same components.
Similar copolymers of butadiene and piperylene prepared with a catalyst system consisting of ethyl aluminum sesquichloride (Et.sub.3 Al.sub.2 Cl.sub.3) and V(AcAc).sub.3 in which the butadiene was exclusively enchained as the 1,4-trans-stereoisomer have been reported by H. Weber et al in Makromol. Chem. 101, 320 (1967).
U.S. Pat. No. 3,642,758 discloses a catalyst system comprising TiX.sub.3.nAlI.sub.3 -xAlR.sub.3 and a Lewis Base, useful for the stereospecific polymerization of butadiene. Related catalyst systems have been disclosed in U.S. Pat. No. 3,245,976; U.S. Pat. No. 3,668,450; U.S. Pat. No. 3,779,944; Belgian Pat. No. 551,851; Belgian Pat. No. 602,496; British Pat. No. 1,138,840; and U.S. patent application, Ser. No. 392,966, filed Aug. 30, 1973, which is herein incorporated by reference.